A conventional oil condition detecing apparatus is known which is of the type described in, for example, Japanese patent application Public-Disclosure No. 61-20851. This conventional apparatus is arranged to detect any potential difference occurring between a reference electrode and a metal electrode, both of which are in contact with a lubricating oil, and to determine the extent of deterioration of the lubricating oil by comparing the potential difference with a refenence value. Further, this apparatus includes a temperature sensor for detecting the temperature of the lubricating oil for the purpose of providing for temperature compensation.
In this prior art, however, since detection of variations (or deterioration) in the oil condition is performed in such a manner that any variation of the electrostatic capacitance between the electrodes caused by the deterioration of the oil is obtained as a change in voltage and this change in voltage is directly measured, there are certain problems, as described below.
Firstly, the change in voltage that occurs between the electrodes which depends on the variation of the capacitance caused by the deterioration of the oil is very small. In particular, when the size of the electrodes is made relatively small in order to allow this apparatus to be adapted for use with engines for vehicles, the voltage change would become even smaller (the difference between the capacitances in the normal oil condition and in the deteriorated oil condition is usually in the range of 1-2 pF, the voltage change being proportional to this). Since detection of such a very small change suffers due to the poor S/N ratio, such very small changes in voltage would beoome undetectable under the influence of the inductance and stray capacitance that is involved with the leads extending from a sensor to a processing circuit if the length of the leads is substantial. It has therefore been difficult to put this conventional type of apparatus into practical use.
Additionally, accurate detection of such very small changes in voltage by the use of a conventional apparatus would require very large electrodes and this would in turn lead to the necessity for a large apparatus. Therefore, the conventional apparatus has very limited adaptability in general-purpose applications.
An object of the present invention, therefore, is to solve the problems mentioned above and to provide an oil condition detecting apparatus which can achieve accurate detection of the state of an oil with relatively compact apparatus.
Another object of the present invention is to provide such an oil condition detecting apparatus which is capable of achieving accurate detection of the extent of deterioration of an oil.